Hi Lachu. A client computer cannot contact a domain controller (dc) when the dc is disconnected from the network. However if you have logged onto this workstation before, it will still let you logon with the user credentials it cached from the previous login. This is assuming you only have one dc.
If you have two dcs then the client will automatically communicate with the secondary dc by querying the DNS server for the appropriate DNS records to find the second domain controller.
You can read a more in depth technical explanation about how this works at Microsoft’s web site: Domain Controller Locator.